This invention relates to joint endoprostheses and, more specifically, to joint endoprostheses having one prosthetic element fixed to a first bone, e.g., a distal femur, and a second prosthetic element fixed to a second bone, e.g., a proximal tibia, each of the prosthetic elements having surfaces of engagement which are in contact with each other particularly during relative movement between the two bones.
It is generally recognized that there are three general categories of contact between the articulating surfaces of joint endoprostheses; theoretical point contact, theoretical line contact and area contact. Theoretical point contact occurs where adjacent bodies, not under load, i.e., undeformed, are touching at a point. Theoretical line contact occurs where adjacent bodies not under load, are touching along a line. Theoretical area contact occurs when adjacent bodies, not under load, are touching over an area. It is also recognized that theoretical contact ordinarily is not achieved because most bodies in engagement are under load which causes deformation of the material in contact.
More specifically, it will be recognized that when prostheses are subject to loads, although there may be described to occur theoretical point contact between two engaging surfaces, there factually occurs deformation which results in the actual contact being over an area around the theoretical point which area may be referred to as being a contact configuration. The same is true with respect to theoretical line contact. Further, the closer that undeformed engaging surfaces are to being congruent, the greater the area contact which occurs as a result of deformation under load. For purposes of this application, the reality is that there is no true point contact, that there is no true line contact, and that areas of contact in theoretical point and line contacts increase under load as the configurations of engaging surfaces approach congruency, will not only be recognized but will be used to advantage.
With the exception of the mobile bearing joint prosthetics disclosed in our prior U.S. patents, e.g., U.S. Pat. No. 4,470,158, U.S. Pat. No. 4,309,778 and U.S. Pat. No. 4,340,978, as well as U.S. Pat. No. 4,085,466, most known joint endoprostheses are typified by highly incongruent contact. The less congruency the greater the contact stresses which are produced under load. Often the stresses experienced in known joint prostheses exceed the acceptable limits associated with known bearing materials such as ultra high molecular weight polyethylene (UHMWPe) which is normally used in these applications. This problem is discussed in detail in our study which was published in 1987. Pappas, M. J., Marcus, G. and Buechel, F. F. "Evaluation of Contact Stresses in Metal-Plastic Total Knee Replacements," Biomaterials and Clinical Applications, p. 259, Elsevier Science Publishers B. V. Amsterdam, 1987.
Known joint prosthetics having fixed bearing surfaces as distinguished from mobile bearing designs, experience high levels of incongruency. Incongruent contact is inherent in such fixed bearing designs since knee motion is highly mobile and includes flexion-extension, axial rotation, anterior-posterior translation, and adduction-abduction. The long standing dilemma for designers of fixed bearing knees is to effect a compromise between the conflicting requirements for joint mobility (which is accomplished by increasing surface contact incongruity and thus contact stress) and low contact stress (which requires high congruity and thus low joint mobility) to prevent rapid failure of the plastic used in current prosthetic joint articulations. The generally accepted approach to resolving this dilemma has been to design prostheses which mimic as closely as possible, the design and operation of the natural knee.
Unfortunately, a satisfactory compromise has yet to be found where fixed bearing knee components can be considered safe for extended use under normal physiological loads. For current metallic and plastic materials used in joint replacement, only mobile bearing elements are capable of normal knee motion with contact stresses such that the plastic bearings appear capable of supporting normal physiological loads over an extended period of time.
There are patients, however, whose ages and/or infirmities are such that loads which they might produce in their generally restricted day-to-day activities are significantly less than the loading to be expected from a normal adult. These lower loading levels which produce lower contact stresses for a given articulation geometry, coupled with the reduced time and frequency of use ordinarily associated with the aged or infirm which reduce the likelihood of bearing damage for given contact stresses, permit the use of articulating surfaces which have a greater degree of incongruity, e.g., prostheses having fixed bearing components.
As compared to mobile bearing prostheses, fixed bearing prostheses need not utilize a supporting prosthetic platform, i.e., the bearing element can be fixtured directly to the bone. By eliminating the relatively expensive supporting platform, the cost of the prosthesis is reduced substantially, a clearly desirable benefit in these days of continuing efforts to reduce medical costs while producing adequate care. Since a low cost, fixed bearing device can be used in joint prostheses, e.g., as tibial or patellar components of a total knee in an elderly, inactive or infirm patient, the added costs of multi-part tibial or patellar replacements often are not justifiable if a lower cost design is adequate.
Thus there has been a need for a joint endoprosthesis which is structurally and operationally adequate and safe for lower load applications and which is lower in cost than known joint prosthetics which are capable of handling typical loading from active adult patients. There also is a need for such low-cost joint endoprostheses which are suitable for use where there is no retained cruciate ligamenture and where such ligamenture is retained. It is the object of the invention to produce such prostheses in the context of a fixed bearing endoprosthesis.